What is a leap second?
A leap second is an extra tick sometimes added to Coordinated Universal Time, or UTC, so civil clocks stay close to time based on Earth’s rotation. Atomic clocks keep a steadier rhythm than the planet does: Earth speeds up and slows down slightly, and over time that creates a gap between clock time and solar time. Leap seconds correct that gap. They are different from leap years, which adjust the calendar to keep seasons in the right part of the year. A leap second adjusts the clock, and it can be awkward for computer systems that expect time to move in a simple, continuous order.
Why leap seconds exist
Modern timekeeping rests on two ideas that do not quite match.
Atomic time comes from physical processes that can be measured with great consistency. It gives the world a stable base for communication, navigation, science, finance and computing. Civil time, though, also has to make sense to people standing on Earth. Noon should remain broadly connected to the Sun being high in the sky.
The problem is that Earth’s rotation is not perfectly regular. It changes because the planet is a physical body, affected by tides, its interior, oceans, atmosphere and other forces. These changes are small, but timekeeping is precise enough to notice them.
A leap second is the correction used when UTC needs to stay aligned with astronomical time. It keeps human clock time tied to the rotating planet, rather than letting atomic time run on without regard to the sky.
How a leap second is inserted
A leap second is added at a UTC day boundary. Instead of moving straight from the final labelled moment of one day to the first labelled moment of the next, UTC includes an extra labelled second in between.
Most people never see this happen. Phones, laptops, network services and other connected devices usually receive time from systems that apply the adjustment for them. A wall clock or watch may simply be corrected later, or may be close enough that the change is not noticeable.
The key point is that a leap second is not like a leap year. A leap year adds a calendar day on a predictable schedule. A leap second is an irregular clock adjustment, based on measurements of Earth’s rotation. It happens only when timekeepers decide the gap needs correcting.
Why computers find leap seconds hard
Computers like time to behave neatly. Many systems assume that each minute has the same structure, that each timestamp is unique, and that time always moves forward. A leap second can break those assumptions.
The trouble is not the size of the adjustment. It is the shape of it. An extra labelled tick can make timestamps appear to repeat, pause or arrive out of order. That matters for databases, payment systems, trading systems, security logs, distributed networks and any software that compares events by time.
Engineers use different methods to reduce the risk. Some systems apply the leap second directly. Others spread the adjustment gradually over a wider period, so clocks slow down or speed up slightly instead of stepping sharply. That spreading method can make systems easier to run, but it also means not every clock agrees in exactly the same way during the adjustment.
The future of leap seconds
Leap seconds may be phased out in the future because the practical cost has grown. The world now depends on systems that exchange precise timestamps across borders, networks and machines, and irregular clock adjustments are difficult to handle consistently.
Removing leap seconds would not make Earth rotate more evenly. It would mean accepting a slow drift between atomic clock time and solar time, or replacing the current practice with a different kind of correction. The trade-off is simple: keep civil time closely tied to Earth’s spin, or make global timekeeping easier for technical systems.